As of Debian 5.04 (Lenny), Debian doesn’t support mounting ext4 partitions out of the box, or rather, we should say, using conventional methods.
Trying to mount an ext4 partition using the mount directive yields the error :
mount: unknown filesystem type 'ext4'
The roundabout way of doing it is setting a test_fs flag on the partition using the tune2fs utility.
tune2fs -E test_fs /dev/hda1
mount -t ext4dev /dev/hda1 /place/to/mount/
Replace hda1 with the name of the ext4 partition you want to mount.